Astronomers used coordinate systems to find and map
objects. Because Earth's time is not exact,
Astronomers rely on Sidereal time.

A sidereal day
is two successive upper meridian crossings by the
Sun of the Vernal Equinox - of course and easier way
to remember this is that a sidereal day is measured
by a complete Earth rotation with respect to the
stars (not the
Sun which would be a standard Solar
day).

Vernal Equinox is also known as the first day of
Spring - or (around) March 21.

Midnight at the Vernal Equinox is 00:00 hours local
Sidereal time.

It is also important to understand that
Astronomer's often use different times than we are
used to - for example our watches are in sync with
the
Sun. This means that one day is equal to Earth's
rotation with respect to the
Sun. This is called a
Solar day. For more accuracy, Astronomer's use
Sidereal time, Earth's rotation with respect to the
stars. This means that the same constellation
appears at the same place after a complete rotation
of the Earth. When comparing a Solar day to a
Sidereal day, subtract 4 minutes from each other:

Sidereal Day = Solar Day - 4
minutes

This is why the night sky is
different depending on the time of year, they rise 4
minutes earlier each night. So why 4 minutes?

A complete circle is 360°
and Earth orbits the
Sun in 365.25 days, or
about 1° a day.

An Earth rotation is also 360
which is 24 hours (or 1440 minutes). Divide 360°
into 1440 minutes and there is 4 minutes left
over. To match with Sidereal time, the Earth
must rotate 1° more
for the
Sun to appear in the same spot day by
day.

A Sidereal Month is also used, and is the orbit of
the
Moon with respect to the
stars which is 27.3
days. A Synodic Month is the "standard"
Moon orbit
about the Earth - from full
moon to full
moon -
which is 29.5 days.